Malaria is a devastating disease that causes widespread morbidity and mortality in areas where it is transmitted by anopheline mosquitoes. In areas of high transmission young children and non-immune visitors are most at risk from this disease, which is caused by protozoa of the genus Plasmodium. In areas of lower or unstable transmission, epidemics of the disease can result and afflict individuals of all ages. The most dangerous form of malaria, responsible for much of the morbidity and most of the mortality, is caused by the species Plasmodium falciparum. It has been estimated that 2 billion people are at risk from malaria, with 200–300 million clinical cases and 1–2 million deaths each year.
The parasite has a complex life cycle in its human and mosquito hosts. In humans the stage of the life cycle which is responsible for the clinical symptoms of the disease occurs in the bloodstream. During this phase the parasite is largely hidden within host red blood cells. Here the parasite grows and multiplies. For example, within a red blood cell each P. falciparum parasite divides several times to produce approximately 20 new ones during a 48 hour cycle. At this point the red blood cell is burst open and the parasites (called merozoites at this stage) are released into the bloodstream. The merozoites must enter new red blood cells in order to survive and for the cycle of replication in the blood to continue. If the parasites do not manage to enter red blood cells they cannot survive for very long and are rapidly destroyed. Symptoms of malaria such as fever are associated with this cyclic merozoite release and re-invasion of red blood cells.
There is an urgent need for a vaccine against malaria. There is no effective vaccine currently available. In addition, mosquito control by the spraying of residual insecticides is either becoming ineffective or considered to be unacceptable, and there is a very worrying spread of drug resistance within parasites. The rapid spread of drug resistance is worrying because compounds such as the cheap and once-effective chloroquine are no longer useful in many parts of the world, and there are few if any new drugs available that are both cheap and effective. Vaccines against microorganisms can be very cost effective and efficient ways to protect populations against infectious diseases.
Because of the complexity of the parasite's life cycle there are a number of points in its development within humans that could be the target of a protective immune response. It is known that with increasing age and exposure individuals do become immune to malaria, suggesting that protective responses do develop with time. Broadly speaking there are three types of vaccine strategy: to target the pre-erythrocytic stages, the asexual blood stage and the sexual stage. The pre-erythrocytic stages are the sporozoites that are injected by an infected mosquito when it takes a blood meal and the initial development of the parasite in the liver. The asexual blood stage is the infection and release of merozoites from red blood cells that occurs in a cyclic manner, and the stage responsible for the manifestation of the clinical symptoms. The sexual stage takes place in the mosquito's gut after it has ingested gametocytes in a blood meal and this initiates the infection of the insect to complete the cycle; a vaccine against the sexual stages would not protect the individual but could reduce transmission and therefore the incidence of malaria in a given human population.
During the asexual cycle in the blood the parasite is directly exposed to the host's immune system, and in particular to antibodies circulating within the bloodstream, only transiently: when merozoites are released by rupture of one cell and before they penetrate another. If there are specific antibodies that can bind to the surface of the parasite then it is possible that these antibodies will interfere with the ability of the parasite to invade a new red blood cell. In fact it has been shown that several monoclonal antibodies that recognise single epitopes on parasite surface proteins, are capable of neutralising the parasite and preventing the cycle of reproduction within red blood cells.
One of the best characterised proteins on the surface of the merozoite is called merozoite surface protein 1 (MSP-1). MSP-1 is a large protein that varies in size and amino acid sequence in different parasite lines. It is synthesised as a precursor molecule of ˜200 kDa by the intracellular parasite and located on the parasite's surface. During release of merozoites from red blood cells and the re-invasion of new erythrocytes the protein undergoes at least two proteolytic modifications. In the first modification as a result of a process called primary processing, the precursor is cleaved to four fragments of ˜83, 30, 38 and 42 kDa that remain together as a complex on the merozoite surface. This complex also contains two other proteins of 22 kDa and 36 kDa derived from different genes. The complex is maintained by non-covalent interactions between the different subunits and is held on the merozoite surface by a glycosyl phosphatidyl inositol anchor, attached to the C-terminus of the 42 kDa fragment and inserted into the plasma membrane of the merozoite. At the time of merozoite invasion of an erythrocyte the C-terminal 42 kDa fragment is cleaved by a second proteolytic cleavage in a process called secondary processing. The result of secondary processing is that the entire complex is shed from the surface of the merozoite except for a C-terminal sub-fragment that consists of just under one hundred amino acids and which is carried into the newly invaded erythrocyte on the surface of the merozoite.
Based on sequence similarities, the structure of this small C-terminal fragment (called MSP-119) was suggested to consist of two epidermal growth factor (EGF)-like domains (see sequence in FIG. 1) (Blackman et al., 1991). An EGF-like motif consists of a 45-50 amino acid sequence with a characteristic disulphide bonding pattern and such domains occur frequently in extracellular modular proteins of animals. In the MSP-1 C-terminal fragment each of the motifs contains six Cys residues proposed to form three disulphide bonds and each motif has a partial match to the EGF consensus (see FIG. 1). However, because the degree of similarity is limited and since the pattern of its disulphide bonding is not known, the designation of the MSP-1 C-terminal fragment as comprised of EGF-like structures has been regarded as tentative. Other relatively divergent potential EGF-like sequences occur in Plasmodium proteins, but previous structure determinations have been confined to those from metazoan organisms (Campbell et al., 1998).
A number of studies have implicated MSP-1 as the target of a protective immune response. Although the goal of this work is to develop a malaria vaccine for use in humans, out of necessity most of this experimental work has been done either in model animal systems or in vitro. These include studies of the effect of specific antibodies on parasite invasion of erythrocytes in vitro, passive immunisation studies in rodent malaria models in laboratory mice and direct immunisation in both rodent and primate malaria models using either native protein (derived from the parasite) or recombinant protein expressed from parts of the MSP-1 gene in heterologous organisms. Sero-epidemiological studies have also showed a correlation between human antibody responses to parts of the MSP-1 molecule and protection against clinical disease. Much, but not all, of the work has focused on the immune response to the C-terminal MSP-119. For example some monoclonal antibodies that recognise MSP-119 prevent red blood cell invasion in in vitro cultures (Blackman et al., 1990). Interestingly, these antibodies that inhibit invasion also inhibit the secondary processing of the 42 kDa fragment, suggesting the mechanism by which they work is by steric hinderance of the protease responsible for secondary processing (Blackman et al., 1994). Since secondary processing goes to completion during successful invasion, if it cannot occur then invasion is interrupted.
All of the work described above would suggest that MSP-1 and in particular polypeptides based on the C-terminal sequence that forms the 42 kDa or the MSP-119 region, should be very good candidates for malaria vaccine development. However, several studies have shown that the epitopes or binding sites for antibodies on MSP-119 require a correct polypeptide tertiary structure, and that this is destroyed by treatments that reduce the disulphide bonds that are postulated to be present between the cysteine residues present in MSP-119. This limitation appears to have been overcome by the expression of recombinant protein in ways that allow antibodies that recognise the native parasite MSP-1 to bind. Other investigators have suggested that other parts of MSP-1 also have potential for inclusion in a vaccine, however the MSP-1 C-terminal fragment is currently the lead candidate for development of a vaccine against the blood stages of the malaria parasite (Diggs et al., 1993; Stoute et al., 1998).
As stated above, every ˜48 hours P. falciparum merozoites are released from the infected erythrocyte to re-invade new red blood cells and during this time they are exposed to the host's immune system. Therefore, the question arises as to how the parasite has evolved to avoid the potentially lethal effects of, for example, neutralising antibodies. In other infectious micro-organisms it is clear that there is a constant battle between the immune system and the micro-organism, and that sophisticated mechanisms have been evolved by micro-organisms to evade the immune response. For example antigenic variation and antigenic diversity are two mechanisms that involve presenting the immune system with “a moving target” such that even though an immune response to one variant of the micro-organism may kill that variant, new variants are produced that are at least partially or fully resistant to the immune response. In the case of malaria merozoites and in particular MSP-1 an alternative mechanism has been proposed whereby the binding of some antibodies (“blocking antibodies”) can prevent the binding of neutralising antibodies and thereby allow the parasite to successfully invade a red blood cell even in the presence of neutralising antibodies (Guevara Patiño et al., 1997). These blocking antibodies may be of two types, those against epitopes that are formed from amino acids that are distant in the linear primary sequence from the epitopes that are the target of neutralising antibodies, and those that are against epitopes that overlap with the epitopes of the neutralising antibodies. This represents a novel mechanism by which a parasite can evade an effective immune response, and unlike mechanisms based on antigenic polymorphism or diversity, it is not dependent upon amino acid sequence diversity.
Some monoclonal antibodies (mAbs) that bind to MSP-119 inhibit the proteolytic cleavage and erythrocyte invasion, suggesting that cleavage is a prerequisite for invasion (Blackman et al., 1994). Other mAbs that bind to the MSP-1 C-terminal fragment do not inhibit processing or invasion but block the binding of the inhibitory neutralizing antibodies. Other antibodies that bind to MSP-119 neither inhibit nor block the binding of inhibitory antibodies. In the presence of blocking antibodies, inhibitory antibodies are ineffective and invasion proceeds. The balance between inhibitory and blocking antibodies induced by immunisation may be a critical factor in determining whether or not the immune response is effective in preventing invasion (Guevara Patiño et al., 1997).